1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transmitters and/or receivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from the high-frequency field radiated by an antenna of the read and write terminal. The present invention applies to such systems, be they read-only systems, that is, including a terminal only reading the data from one or several transponders, or read/write systems, in which the transponders contain data that can be modified by the terminal.
2. Discussion of the Related Art
Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended for being near-field coupled when the transponder enters the field of the read/write terminal.
FIG. 1 very schematically shows a conventional example of a data exchange system of the type to which the present invention relates between a read/write terminal 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3 (generally, the ground). The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and exploiting received data including, among others, a modulator/demodulator and a microprocessor for processing the control signals and the data. The exploitation of the received data is based on a measurement of the current in the oscillating circuit or of the voltage thereacross. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power necessary to their operation from a supply circuit (not shown) connected, for example, to the electric supply system or to batteries.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of control and processing circuits 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor capable of communicating with other elements (for example, a memory), a demodulator of the signals received from terminal 1, and a modulator for transmitting information to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on the same frequency corresponding to the frequency of an excitation signal of the terminal's oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a transmission carrier but also as a remote supply carrier for the transponder(s) located in the terminal's field. When a transponder 10 is located in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of its resonant circuit. This voltage, after being rectified and possibly clipped, is intended for providing the supply voltage of electronic circuits 13 of the transponder. For clarity, the rectifying, clipping, and supply means have not been shown in FIG. 1. In return, the data transmission from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. The load variation is performed at the rate of a so-called back-modulation sub-carrier, of a frequency (for example, 847.5 kHz) smaller than that of the carrier.
The antennas of terminal 1 and of transponder 10 are, in FIG. 1, materialized by their equivalent electric diagrams, that is, inductances (neglecting the series resistances). In practice, a terminal 1 has a flat antenna L1 formed of a few circular turns (most often one or two turns) of relatively large diameter (for example, of a given value ranging between one and 4 inches) and antenna L2 of a transponder (for example, a card of credit card format) is formed of a few rectangular turns (most often from two to five turns) inscribed within a relatively small diameter (turns with a side from 2 to 3 inches) as compared to the diameter of antenna L1.
FIG. 2 is a simplified perspective view of a terminal and of a transponder illustrating a conventional example of antennas. Electronic circuits 4 of terminal 1, as well as capacitor C1 and resistor R1, are generally contained in base 6. Antenna L1 is, for example, supported by a printed circuit wafer 7 protruding from base 6. In FIG. 2, it is assumed that antenna L1 is formed of a single turn in which, when the terminal's oscillating circuit is excited by the high-frequency signal, a current I flows. The indicated direction of current I is arbitrary and this current is alternating. Transponder 10 is assumed to be a smart card integrating circuits 13 and antenna L2 of which includes two rectangular coplanar turns approximately describing the periphery of card 10. Capacitor C2 shown as separated from circuits 13 is generally formed by being integrated to the chip.
Conventional transponder systems generally have a limited range, that is, at a certain distance (d, FIG. 2) from the terminal, the magnetic field is insufficient to properly remotely supply a transponder. The minimum field generally ranges between 0.1 and 1 A/m according to the transponder's power consumption, which essentially differs according to whether it is or not provided with a microprocessor.
The remote supply range depends on the amount of magnetic flux emitted by the terminal or reader, which can be “intercepted” by a transponder. This amount directly depends on the coupling factor between antennas L1 and L2, which represents the flux proportion received by the transponder. The coupling factor (between 0 and 1) depends on several factors among which are the mutual inductance between antennas L1 and L2 and the respective size of the antennas, and the tuning of the oscillating circuits on the high-frequency carrier frequency. For given sizes and a given mutual inductance, the coupling is maximum when the oscillating circuits of the terminal and of the transponder are both tuned on the frequency of the remote supply carrier.
A conventional solution to increase the range consists of increasing the size of antenna L1 of the terminal. To keep the magnetic field, the intensity of the current of the excitation signal must then be proportionally increased. A first disadvantage of such a solution is that it increases the necessary system excitation power. A second disadvantage of such a solution is that such a current increase remains limited by the generator structure and requires components having significant size (in particular, a large cross-section of the conductor forming antenna L1). Further, the losses are proportional to the square of the current.
To attempt overcoming this second disadvantage, a known solution is to use, for relatively large antennas (for example, of portico type), a parallel oscillating circuit on the terminal side. This circuit is then voltage-driven and no longer current-driven, which results in a greater increase of the current in the antenna (assembled as a so-called “rejector” circuit) without requiring this current to flow through the generator. Such a solution has the advantage of limiting losses. However, this solution still causes an increase in the power consumption (due to the voltage increase to increase the power). Further, the maximum field at the center of antenna L1 is generally set by standards.
Another disadvantage, mostly present for antennas of relatively large size, is that the magnetic field is not homogeneous in front of the antenna, that is, for a given distance, the intensity of the magnetic field strongly varies according to the position in a plane parallel to the antenna. This disadvantage of course cumulates with the foregoing when the range is desired to be increased by increasing the size of the antenna, that is, the surface area in which it is inscribed.
U.S. Pat. No. 5,142,292 discloses an antenna including a plurality of series-connected coils for transmitting electromagnetic energy.